The increasing number of connections to be welded on electronic cards and, especially, the development of electronic cards produced in small batches have promoted the development of machines for reflow soldering in vapor phase. When soldering in vapor phase, the solder metal is deposited, in an initial stage, in the form of paste or solid solder preform. The soldering as such is performed in a second stage by the addition of heat. The heat is obtained by the condensation of primary liquid on the component. The primary liquid is heated to its boiling point which is above the melting point of the solder, so as to obtain a zone of saturated vapor. At atmospheric pressure, the temperature of the saturated vapor zone is the same as the boiling temperature of the liquid. Since the entire surface of the object to be soldered is in contact with the vapor, this vapor is heated uniformly and provides for the simultaneous soldering of all the connections that have to be made.
The part or component to be welded is plunged in a primary vapor zone. This vapor condenses upon contact with the part, thus providing for the addition of heat and for the melting of the previously deposited solder.
With a view to reducing losses of primary liquid through convection, diffusion or leakage, a secondary vapor zone forming a barrier is created, using a liquid which is often of greater density and lower boiling temperature and is less costly.
A liquid frequently used as a primary liquid is, for example, the one marketed by the 3M Company under the brand name "Fluorinert" with the reference FC 70. The secondary liquid used may be, for example, trifluorotrichloroethane with the formular CFCl.sub.2 --CCIF.sub.2. The method of vapor phase soldering uses liquids containing fluorine which are heated to high temperatures, for example 215.degree. C. At these temperatures, the primary and secondary liquids tend to decompose partially, the secondary liquid releasing Cl.sup.- and F.sup.- ions. The presence of these ions will cause two types of corrosion: firstly, a dry corrosion through the formation of the chloride and fluoride of the affected metals, and secondly an aqueous corrosion through the formation of acid in the presence of water. The presence of chlorine and fluorine ions has two consequences: firstly the corrosion of the vapor phase soldering machine, and secondly the presence of ions on the soldered parts, possibly leading to corrosion.